To reduce man-hour required to mount a detector inside a fluid pressure cylinder without mounting the detector outside the fluid pressure cylinder.

A wire winding part 19 is mounted on a detecting chamber 18 in a cylinder tube 12, and a supporting shaft 22 rotatably-supported on a spool supporting part 20 is energized so as to rotate in such a direction that wire 26 is wound by a spiral spring 23. The wire 26 is wound around a spool 21 fixed to the supporting shaft 22 so that it may be incapable of relative rotation, and the end of the wire 26 is connected to a piston 14. A flange 25b of the spool 21 is provided with a plurality of parts 27 to be detected on the identical circumference, and a magnetic sensor 28 capable of detecting the respective parts to be detected is mounted on the cylinder tube 12. A magnetic sensor 29 detecting the piston 14 disposed at the initial position is mounted on the cylinder tube 12. A stroke detecting unit 30 determines the stroke position of the piston 14 based on the number of the parts 27 to be detected for the amount of stroke of the piston 14 in the initial position.